How to make multiple displacements map node
If you paint an image on the model with texture drawing, it just copies all the texture channels into your target texture. It's totally up to you, what kind of information you store in those channels. You can paint even normal maps onto a model.
If the displacement maps are just grayscale images, you mix them with add, or screen (or whatever you want) blending, and then feed the result to the Bump node (or to displacement input of the material).
You even don't need to separate the colors of your painted colored image in the photoshop - you can use 'Converter\Separate RGB' node instead, play with the levels of each channel, mix them together, and feed that mixed value to the Height input of the Bump node, or to the Displacement input of the material output node.
And images on your link are enough to play with to have an idea. If you want, you can paint some scratches with white and black brush (50% gray = no displacement) to see how it affects the look of the surface of your mesh.